Environmental pollution problems, especially air quality degradation, have become a high concern throughout the world and especially in industrial developed countries. Such concerns have led to environmental regulatory policies imposing tight quality regulations on transportation fuels. Of such fuels, diesel fuel is considered to be a major contributor of known harmful pollutants, such as SOx, NOx and particulate matter and, therefore, stringent regulatory standards have been proposed and enacted to reduce the emission of such pollution by diesel fuels.
The sulfur content in fuels is a critical concern, as it is known to form sulfur dioxide when subjected to a combustion process. The sulfur dioxide, together with atmospheric moisture, forms sulfuric acid in the atmosphere. This is the cause of acid rain, which has been attributed to causing substantial damage on the environment as well as man-made structures.
In addition, the generated sulfur oxides have been found to poison noble metal catalysts conventionally used as part of automobile emission after-treatment devices. For this reason, automobile manufacturers have suggested that sulfur content in diesel fuels be reduced to less than 30 weight parts per million (ppm) to meet new tail-pipe emission regulations contemplated to become law. Thus, an ultra-low sulfur diesel (ULSD) market is emerging to replace conventional sulfur diesel fuel standards of 500 ppm. In various countries, such as in the United States and in a number of European countries, regulations have been proposed or enacted to require sulfur content to be reduced to levels of less than 50 ppm and, in certain instances, to levels of less than 15 ppm. In view of the ever-increasing regulatory pressures, petroleum refiners and catalyst producers have invested considerable time, money and effort to produce environment-friendly petroleum products.
Hydrodesulfurization (HDS) processes most commonly used reduce sulfur content in petroleum feedstock by converting sulfur compounds present in the feedstock to hydrogen sulfide. Since the 1960's various HDS processes have been developed which, in general, subject the feedstock to hydrogen under elevated temperatures and pressures in the presence of a catalyst. One mode of reducing the sulfur content is to develop innovative improvements in one or more of the operating parameters of catalyst activity, reaction temperature, bed volume and/or hydrogen partial pressure of the HDS process.
Although catalyst activity has been doubled since HDS catalysts were first introduced, it has been calculated that a factor of 3.2 fold activity improvement is required to meet the present 500 ppm sulfur content and a factor of about 17 is needed to reach the 50 ppm level more highly desired. Thus, if one relies on catalyst activity alone, the number of HDS reactors must be substantially increased and/or, the charge rate substantially decreased unless the catalyst activity is dramatically improved.
As stated above, the reaction temperature can be increased to cause reduction in sulfur content. However, such temperature increase can only be done to a small degree due to the design limitations of present equipment. In addition, very high temperatures are known to cause degradation to the product stream. Similarly, increased pressure would aid in achieving reduced sulfur content but presently designed reactors establish a limit on this parameter, and new equipment capable of handling very high pressures would be costly.
Thus, conventional processes for treating diesel feedstock (also known as light gas oil, LGO) have technical limitations while breakthroughs in catalyst activity have not been realized. Therefore, methods, which use different feedstock instead of LGO, or using innovative reaction pathways, are being studied.
For example, a process developed by Shell Oil Company polymerizes natural gas to produce a distillate composed of C12–C25 products, similar to diesel feedstock. In this process natural gas is converted to syn-gas through a Fischer-Tropsch reaction and the product is polymerized to yield diesel distillate free of sulfur compounds. This process has the drawbacks of using fairly expensive feed and requiring three distinct reaction steps to result in a high cost process.
U.S. Pat. No. 5,454,933 discloses an adsorption process to produce sulfur-free diesel fuel by removing remaining sulfur compounds from LGO material that has already undergone hydrodesulfurization. The disclosed post-HDS process utilizes adsorbents designed to directly remove residual sulfur compounds from post-HDS treated material.
It has been proposed that reduction or removal of nitrogen containing compounds from streams being fed to a catalytic HDS unit causes HDS to take place in a more efficient manner and, thus, make the system capable of producing a product with very low sulfur content using conventional operating parameters.
It is well known that heteroatom containing compounds, particular nitrogen and sulfur containing compounds can be readily removed from light cuts, such as C4–C8 streams, as is obtained from a conventional FCC unit or etherized streams. Different processes, such as adsorption and extraction have been proposed for this purpose. Heteroatom contaminant compounds that are found in such light cut streams are few in number, readily identified, have low molecular weights and have low boiling points consistent with the light hydrocarbons forming this type of cut. As a consequence, these contaminants are easily removed from the feedstream in which they are contained. These features are not applicable with respect to the more complex mixture of heteroatom containing compounds found in heavier hydrocarbon streams. The heavier LGO streams, composed primarily of C12–C30 and higher compounds obtained from distillation or FCC units or the like, contain a vast mixture of heteroatom species. These compounds have been difficult to identify, are generally composed of high molecular weight compounds and have high boiling points. Some of the sulfur species have been identified and studied by Whitehurst et al. in Adv. Catal. 42, 345–471 (1998). Attempts to identify the nitrogen species of such gas oil cuts have been illusive and challenging due to the concentration in the hydrocarbon matrix and the complexity of the mixture of species. A group of scientists from Kyushu University at Fukuoka, Japan and Chevron Research and Technical Company at Richmond, California, have attempted to identify nitrogen containing compounds of gas oils and were only capable of reporting broad classes including alkyl substituted aniline, quinoline and its alkyl derivatives, and, carbazole derivatives (S. Shin et al., Energy & Fuels (2000), 14(3), 539–544. Wiwel et al. in “Assessing Compositional Changes of Nitrogen Compounds of Typical Diesel Range Gas Oils . . . ” (Industrial & Engineering Chemistry Research (2000), 39(2), 533–540) reported that crude oil generally contains from about 0.1 to 2 percent nitrogen compounds but the nitrogen content rapidly increases with increasing boiling point of the oil fraction. Recognizing that diesel fuels are commercially prepared from straight run distillates and cracked products of heavier feedstock, the nitrogen levels normally range from 20–1000 μgN/ml. They report that such compounds are generally made up of four different chemical classes: aliphatic amines, anilines, and five- and six-membered pyridinic ring system compounds. They have identified some 64 compounds (using the method of ASTM D-4629-91) and stated that many more unidentified compounds are contained in this heavier fraction of material.
Removal of nitrogen containing compounds from light cut (C4–C8) petroleum streams has been accomplished because the nitrogen compounds are fewer in number, are readily identifiable and have lower molecular weight. However, because nitrogen containing compounds in heavier fraction material are difficult to identify and, at best, are a complex mixture of compounds, removal has been illusive.
U.S. Pat. No. 2,384,315 discloses filtering crude oil through a bed of bauxite prior to subjecting the oil to catalytic cracking treatment. Such procedure would produce a product still having high amounts of nitrogen compounds relative to today's required standards.
U.S. Pat. No. 2,744,053 discloses the removal of nitrogen compounds from low boiling gasoline hydrocarbon stock by passing the feedstock through an adsorption bed formed from silicon oxide alone or a mixture of silicon oxide and alumina. It is well known that silicon oxide and other conventional adsorbents do not exhibit the Lewis acidity required by the adsorbent used in the present invention.
U.S. Pat. No. 4,708,786 discloses a fluid catalytic cracking process in which the feedstock is treated with a mixture of cracking catalyst and micro-porous refractory oxide capable of sorbing pyridine at room temperature and retaining a portion of the sorbed material. This sorbent is to be used in conjunction with the catalyst in the FCC zone.
U.S. Pat. No. 5,051,163 discloses a process wherein the initial feed to a catalytic cracking reactor is first treated with a small amount of the cracking catalyst. The reference suggests that the nitrogenous material will bind with sacrificial catalyst present in the pre-cracking zone to thus prevent poisoning of the cracking catalyst used in the cracking zone. No suggestion is made as to removal of nitrogenous compounds just prior to hydrodesulfurization that would further decrease the sulfur content after HDS, to enhance the effectiveness of the HDS and to inhibit poisoning of HDS catalyst.
U.S. Pat. No. 5,210,326 and U.S. Pat. No. 5,378,250 are directed to processes which include treating light (C3–C8) hydrocarbon stream obtained from a FCC process zone with a super activated alumina to remove nitrogen compounds, mercaptans and water prior to further processing.
U.S. Pat. No. 6,107,535 and U.S. Pat. No. 6,118,037 also teach processes, which include treatment low molecular weight (C3–C8) hydrocarbon streams with silica gels to remove contaminant compounds that contain sulfur, nitrogen and/or oxygen.
U.S. Pat. No. 6,248,230 discloses a process for manufacturing cleaner fuels by removing natural polar compounds (NPC) from a wide range boiling point petroleum feedstream prior to subjecting the stream to catalytic hydrodesulfurization. The reference teaches that petroleum hydrocarbon product streams obtained from FCC or the like process can be contacted with an adsorbent, such as silica gel, hydrated alumina, activated carbon, active alumina, or clay. The reference states that silica or hydrated alumina are each preferred adsorbent. Such adsorbents are known to be substantially free or have only limited degrees of Lewis acidity. Although this reference indicates that large amounts of the NPC contained in the treated petroleum feedstock can be removed, such removal, especially from an LGO stream, requires uneconomically high ratios of adsorbent to feed.
The above references illustrate the desire by the petroleum refining industry to remove hetero-atom containing compounds from light cut petroleum products. Unfortunately, heavier fraction material, such as diesel fuel fractions have not been successfully treated to remove nitrogen and sulfur containing contaminants commonly found therein in a cost-effective, efficient manner to provide an environmentally friendly product. The removal of organic nitrogen is important to many different refinery processes and is essential to provide a diesel fuel products, which meet the environmental needs and associated regulations being proposed and enacted into law. It is highly desired to provide a cost-effective process to remove a majority or substantially all of nitrogenous compounds from diesel fuel fractions so that the treated diesel fuel feedstream can be effectively and efficiently treated by conventional HDS processes to produce a resultant material having less than 50 ppm and more preferably less than 15 ppm of sulfur containing compounds in the resultant product stream.
An object of the present process is to provide a cost-effective and efficient means of removing nitrogenous compounds from a diesel fuel fraction (C12 and greater, e.g. C12–C30 petroleum feedstream) prior to subjection to HDS treatment.
Another object of the present invention is to provide an economical and efficient means of removing at least about 75 weight percent, preferably at least about 80 weight percent and more preferably at least about 90 weight percent of nitrogenous compounds from a diesel fuel fraction prior to subjection to HDS treatment.
Another object of the present invention is to effectively produce a diesel fuel, which meets present and contemplated environmental regulations with respect to emission of NOx and SOx pollutants.